The ability to create high electric fields near charged carbon nanotubes has stimulated the development of nanotube-based electron field emission sources for a variety of applications, including low-power video displays, atom sensing, and atom interferometry. For example, nanotube-based electron field emission sources could be used in low power video displays and in sensitive gas sensors whose ionization currents depend on the gas pressure and species present in the gas. Typical nanotube-based devices include an array, or forest, of nanotubes or nanowires that extend longitudinally from a substrate. The quantitative behavior of such devices depends on the fact that the large fields used in operation can be obtained at the tips of nanotubes or nanowires in an inhomogeneous forest of nanotubes or nanowires.
Due to shielding effects, active nanotubes in a forest are generally separated from each other by many inactive nanotubes. The effective active area of a forest-based device is thus often a small fraction of the area occupied by the forest. For instance, a forest of nanowires extending over a 1 cm×2 cm area typically includes 1010 nanowires, of which only about 1000 are active in tip ionization. The resulting efficiency of the forest is about 10−7. Furthermore, interpretation of the behavior of forest-based devices is complicated by variability of nanotube lengths, density, and tip geometry, including tip geometry that changes with time, among the nanowires in the forest. As a result, devices based on nanowire forests are often unsuitable for reliable, sensitive atom detection or atom interferometry.